diff --git a/src/config/env-preserve.ts b/src/config/env-preserve.ts
index a882357b9e..0259781eee 100644
--- a/src/config/env-preserve.ts
+++ b/src/config/env-preserve.ts
@@ -1,3 +1,5 @@
+import { isPlainObject } from "../infra/plain-object.js";
+
 /**
  * Preserves `${VAR}` environment variable references during config write-back.
  *
@@ -16,15 +18,6 @@
 
 const ENV_VAR_PATTERN = /\$\{[A-Z_][A-Z0-9_]*\}/;
 
-function isPlainObject(value: unknown): value is Record<string, unknown> {
-  return (
-    typeof value === "object" &&
-    value !== null &&
-    !Array.isArray(value) &&
-    Object.prototype.toString.call(value) === "[object Object]"
-  );
-}
-
 /**
  * Check if a string contains any `${VAR}` env var references.
  */
diff --git a/src/infra/plain-object.test.ts b/src/infra/plain-object.test.ts
new file mode 100644
index 0000000000..b87e555b21
--- /dev/null
+++ b/src/infra/plain-object.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+import { isPlainObject } from "./plain-object.js";
+
+describe("isPlainObject", () => {
+  it("accepts plain objects", () => {
+    expect(isPlainObject({})).toBe(true);
+    expect(isPlainObject({ a: 1 })).toBe(true);
+  });
+
+  it("rejects non-plain values", () => {
+    expect(isPlainObject(null)).toBe(false);
+    expect(isPlainObject([])).toBe(false);
+    expect(isPlainObject(new Date())).toBe(false);
+    expect(isPlainObject(/re/)).toBe(false);
+    expect(isPlainObject("x")).toBe(false);
+    expect(isPlainObject(42)).toBe(false);
+  });
+});
diff --git a/src/infra/plain-object.ts b/src/infra/plain-object.ts
new file mode 100644
index 0000000000..2b611ca33c
--- /dev/null
+++ b/src/infra/plain-object.ts
@@ -0,0 +1,11 @@
+/**
+ * Strict plain-object guard (excludes arrays and host objects).
+ */
+export function isPlainObject(value: unknown): value is Record<string, unknown> {
+  return (
+    typeof value === "object" &&
+    value !== null &&
+    !Array.isArray(value) &&
+    Object.prototype.toString.call(value) === "[object Object]"
+  );
+}
diff --git a/src/utils.ts b/src/utils.ts
index 66ed063cfa..49e14e0d04 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -8,6 +8,7 @@ import {
   resolveEffectiveHomeDir,
   resolveRequiredHomeDir,
 } from "./infra/home-dir.js";
+import { isPlainObject } from "./infra/plain-object.js";
 
 export async function ensureDir(dir: string) {
   await fs.promises.mkdir(dir, { recursive: true });
@@ -54,18 +55,7 @@ export function safeParseJson<T>(raw: string): T | null {
   }
 }
 
-/**
- * Type guard for plain objects (not arrays, null, Date, RegExp, etc.).
- * Uses Object.prototype.toString for maximum safety.
- */
-export function isPlainObject(value: unknown): value is Record<string, unknown> {
-  return (
-    typeof value === "object" &&
-    value !== null &&
-    !Array.isArray(value) &&
-    Object.prototype.toString.call(value) === "[object Object]"
-  );
-}
+export { isPlainObject };
 
 /**
  * Type guard for Record<string, unknown> (less strict than isPlainObject).